Going on a tangent here but why s3? Why not avoid vendor lock in and just serve static files?

My setup is to post blobs over a small rest api which balances storage load over all the file servers using nginx. It responds with the serverSubdomainId which in turn gets saved in the database. So the url for file access can be generated https://{serverSubdomainId}.domain.tld/{resourceId} The only costly part is writing which with caching of available storage of all the servers isn’t bad. The whole system is horizontally and vertically scalable. What am I missing that s3 is still the defacto even with vendor lock in and high egress costs?